A vehicle's fuel systems may supply fuel to an engine in varying amounts during the course of vehicle operation. During some conditions, fuel is not injected to the engine but fuel pressure in a fuel rail supplying fuel to the engine is maintained so that fuel injection can be reinitiated. For example, during vehicle deceleration fuel flow to one or more engine cylinders may be stopped by deactivating fuel injectors. If the engine torque demand is increased after fuel flow to the one or more cylinders ceases, fuel injection is reactivated and the engine resumes providing positive torque to the vehicle driveline. However, if the engine is supplied fuel via direct fuel injectors and a high pressure fuel pump, the high pressure pump may degrade when fuel flow through the high pressure pump is stopped while the fuel injectors are deactivated. Specifically, the lubrication and cooling of the pump may be reduced while the high pressure pump is not operated, thereby leading to pump degradation. Besides deceleration, a direct injection fuel system may periodically cease operation because a different set of fuel injectors are supplying the engine with fuel (as may be the case with a bi-fuel engine). Also, if an electric motor is handling the vehicle's torque needs, fuel injection may cease during that operational mode.
The inventors herein have recognized the above-mentioned issue may be at least partly addressed by a method of operating a direct injection fuel pump, comprising: regulating a pressure in a compression chamber of the direct injection fuel pump to a limited pressure during a direct injection fuel pump compression stroke, the pressure greater than the pressure on the low pressure side of the piston. This pressure limit may be the output pressure of a low pressure pump supplying fuel to the direct injection fuel pump. In one example, a pressure relief valve may be included upstream of the compression chamber of the direct injection fuel pump to regulate the pressure within the compression chamber. However, the pressure relief valve may cause heating of fuel upstream of the direct injection fuel pump. Fuel heating may reduce lubrication of the direct injection fuel pump and may increase power consumption. Accordingly, another method of operating a direct injection fuel pump is provided, comprising: while a solenoid activated check valve at an inlet of the direct injection fuel pump is commanded to a pass-through state during a direct injection fuel pump compression stroke, adding a pre-loaded accumulator upstream of the solenoid activated check valve, the pre-loaded accumulator having a substantially constant pressure-volume characteristic. The pre-loaded accumulator with the substantially constant pressure-volume characteristic may reduce fuel heating.
By regulating pressure in the compression chamber of a direct injection fuel pump it may be possible to lubricate the direct injection fuel pump's cylinder and piston when flow out of the direct injection fuel pump to fuel injectors is stopped. Specifically, a fuel pressure differential across the direct injection fuel pump's piston may be provided that allows fuel to flow into the piston/bore clearance and lubricate an area. Further, pressure in the compression chamber is less than pressure in the fuel rail so there is no flow from the direct injection fuel pump to the fuel rail. In this way, the piston may continue to reciprocate within the direct injection fuel pump with a low rate of degradation and without supplying fuel to the engine.
The present description may provide several advantages. Specifically, the approach may improve fuel pump lubrication and reduce fuel pump degradation. Additionally, pressure in the compression chamber can be regulated to a higher pressure than low pressure fuel pump pressure so that engine operation may be improved during conditions of direct injection fuel pump degradation. Further, the approach may be applied at low cost and complexity. Further still, the approach may reduce fuel pump noise since a solenoid activated check valve at an inlet of the direct injection fuel pump may be deactivated when fuel flow to the engine is stopped. Additionally, several embodiments of direct injection fuel pumps and fuel systems are presented in the Detailed Description below that include accumulators, check valves, and other components and modifications that may create better pump performance while alleviating problems such as pump reflux, noise pollution, and pump degradation caused by inadequate pump lubrication. Adding check valves and accumulators to fuel systems may reduce the adverse effects associated with pump reflux, such as increased stress to the system as well as unnecessarily increased pumping pressure. Furthermore, including an accumulator to the direct injection fuel pump may aid in reducing pump noise while maintaining sufficient lubrication of the pump. Further still, by selecting a pre-loaded accumulator with a substantially constant pressure-volume characteristic, fuel heating may be reduced.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.